The lumbar spine allows primarily flexion and extension, while allowing little axial rotation or lateral bending. Dynamic fixation allowing the patient to make natural adjustments to flexion and extension would allow a patient to maintain good sagittal balance. Conversely, fusion at lumbar levels can lead to problems with sagittal balance and degeneration of adjacent levels that are trying to compensate, especially if the index level is fused in the wrong sagittal balance. Maintenance of sagittal balance and prevention of adjacent level degeneration are advantages that proponents of lumbar dynamic fixation cite. The facet joints in the lumbar spine are oriented vertically, blocking anteroposterior translation, limiting axial rotation, and forcing a particular axis of rotation during flexion and extension that is fairly well focused in the posterior two-thirds of the disc. As such, a device allowing only focused flexion and extension while disallowing all other motions (lateral bending, axial rotation, and translation in any direction) would be an ideal lumbar dynamic device.
Current methods of dynamic stabilization in the spine include artificial discs inserted from an anterior or anterolateral approach to restore the function of the natural disc, e.g., Maverick (Medtronic Inc.); ProDisc (DePuy Synthes); Charité (DePuy Synthes), and posterior dynamic stabilization, such as pedicle-screw-based articulating or bending systems, for restoring the function of the facet joints, e.g., Dynesys (Zimmer Spine) and Acadia (Globus Medical). Since degeneration often affects both anterior and posterior columns simultaneously, it would be desirable to dynamically stabilize the spine in both the anterior and posterior columns. However, it is challenging to design anterior and posterior devices that work together to restore the function of the natural disc and facet joints simultaneously.